Control panels and terminal devices (e.g. slave units, fire sensors, fire detectors, intrusion detectors, fire protection means, smoke venting means, fire extinguishing means) of fire/security protection systems such as fire alarm systems and security systems are equipped with transmission circuits, and it is necessary to check if the transmission circuits have properly transmitted their predetermined signals.
The following method is considered to be suitable for this check. The signal which should be transmitted from the transmission circuit is stored in the memory circuit, and the signal which has been transmitted from said transmission circuit is received by the receiving circuit. This received signal is compared with the signal stored in the memory circuit, and if they match, it means that the transmission circuit has properly transmitted the signal.
Nevertheless, if a trouble occurs in the receiving circuit, it is not possible to detect the signal transmitted from the transmission circuit, and descrimination means such as a CPU judges that no signal has yet been transmitted from the transmission circuit. Therefore, even after the parallel-serial converter has completely sent the signal to be transmitted off to the transmission circuit, the CPU still keeps the transmission circuit in a state ready for signal transmission, and consequently the signal line remains occupied by the terminal device. Therefore, there is such a problem that the control panel or other slave units can not transmit signals to the signal line.